if a board/$(VENDOR)/common/Makefile exists, build it.
also add the first such case, board/freescale/common/Makefile, to
handle building board-shared EEPROM, PIXIS, and MDS-PIB code, as
dictated by board configuration.
thusly get rid of alternate build dir errors such as:
FATAL: can't create /work/wd/tmp/u-boot-ppc/board/freescale/mpc8360emds/../common/pq-mds-pib.o: No such file or directory
by putting the common/ mkdir command in its proper place (the common
Makefile). Common bits from existing individual board Makefiles have
been removed.
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
